EXCHANGE 


A  STUDY  OF  THE  EFFECT  OF  ADSORBED 

GAS   ON    THE    HIGH-FREQUENCY 

RESISTANCE  OF  COPPER  WIRE 


A  THESIS 

PRESENTED  TO  THE  FACULTY  OF  THE  GRADUATE  SCHOOL 

OF  CORNELL  UNIVERSITY  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY 


BY 
AUSTIN  BAILEY 


Reprinted  from  PHYSICAL  REVIEW,  Vol.  XX,  No.  2,  August,  1922. 


A  STUDY  OF  THE  EFFECT  OF  ADSORBED 

GAS   ON    THE    HIGH-FREQUENCY 

RESISTANCE  OF  COPPER  WIRE 


A  THESIS 

PRESENTED  TO  THE  FACULTY  OF  THE  GRADUATE  SCHOOL 

OF  CORNELL  UNIVERSITY  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY 


BY 
AUSTIN  BAILEY 


Reprinted  from  PHYSICAL  REVIEW,  Vol.  XX,  No.  2,  August,  1922. 


\\ 


[Reprinted  from  THE  PHYSICAL  REVIEW,  N.S.,  Vol.  XX,  No.  2,  August,  1922.] 


A  STUDY   OF  THE   EFFECT   OF   ADSORBED    GAS    ON   THE 
HIGH-FREQUENCY   RESISTANCE   OF  COPPER  WIRE. 

BY  AUSTIN  BAILEY. 

SYNOPSIS. 

Effect  of  Adsorbed  Gas  on  the  High-frequency  Resistance  of  a  Loop  of  Copper  Wire. 
— This  resistance  is  known  to  be  greater  than  would  theoretically  be  expected  if  the 
surface  layers  have  the  same  resistivity  as  the  core.  The  experiments  described 
were  undertaken  to  determine  whether  this  increased  resistance  is  due  to  gas  adsorbed 
by  the  surface  layer  of  oxide.  A  square  loop,  60  cm.  on  a  side,  was  formed  by 
enclosing  copper  wires  inside  of  glass  tubes  connected  to  a  vacuum  pump,  and  after 
several  hours'  preliminary  heating  and  pumping,  the  wire  was  glowed,  and  then 
its  resistance  for  radio-waves  of  20  meters  length  was  found  to  increase  in  20  minutes 
by  several  per  cent.,  though  the  residual  pressure  was  only  io~6  atm.  After  suc- 
cessive glowings  the  effect  became  progressively  less  as  the  oxide  layer  disappeared. 
With  higher  pressures  the  increase  to  an  equilibrium  value  came  more  rapidly. 
These  results  indicate  that  the  gas  adsorbed  by  the  copper-oxide  layer  does  increase 
the  resistance,  and  suggest  that  possibly  copper  wire  whose  surface  is  covered  with 
a  thin  layer  of  metal  such  as  tin,  might  have  a  lower  high-frequency  resistance  than 
oxidized  copper  wire  of  the  same  size. 

Short  radio-waves,  of  less  than  20  meters  were  produced  by  using  only  the  capacity 
of  the  oscillating  triode  tube  in  the  high-frequency  circuit.  The  wave-length  was 
measured  to  within  f  per  cent,  by  use  of  a  modified  Lecher  System,  with  a  hot-wire 
galvanometer  at  a  current  loop.  Currents  in  the  square  loop  were  measured  by 
means  of  a  loosely  coupled  circuit  containing  a  special  vacuum  thermo-element 
with  minimum  coupling  in  the  galvanometer  circuit. 

Adsorption  of  Gases  by  Copper  Oxide  and  Copper. — The  observations  indicate 
that  copper  oxide  readily  absorbs  gases  at  room  temperature  and  liberates  them 
when  glowed,  while  copper  does  not. 

An  arc-type  diffusion  pump  is  described,  which  has  an  iron  wire  ballast  resistance 
wound  around  the  tube  which  conducts  the  vapor  to  the  condensation  chamber. 
This  enables  the  pump  to  get  into  operation  quickly.  • 

INTRODUCTION. 

A  T  very  short  wave-lengths,  measurements  made  of  the  high  fre- 
*•*  quency  resistance  of  a  one  meter  square  loop  of  wire  show  a  much 
larger  value  than  that  calculated  irom  the  formula  for  the  resistance  of  a 
straight  wire  of  the  same  length.  The  comparison  between  the  obseived 
and  calculated  values  of  the  resistance  of  a  one  meter  square  loop  of 
No.  18  B.  &  S.  gauge  copper  wire  are  shown  in  Fig.  I. 

This  discrepancy  may  be  due  to  a  number  of  causes,  one  of  which  may 
be  gas  adsorbed  on  the  surface  of  the  wire.  Langmuii1  has  pointed  out 
that  there  is  probably  a  very  close  packing  of  the  gas  molecules  adsorbed 

1  I.  Langmuir,  PHYS.  REV.,  8,  149  (1916). 


155 


AUSTIN    BAILEY. 


[SECOND 

[SERIES  . 


on  the  surface  of  a  metal.  Such  a  condensation  would  doubtless  alter 
materially  the  physical  properties  of  the  surface.  Since  at  high  fre- 
quency, a  large  part  of  the  current  will  flow  in  a  thin  skin  near  the  surface, 
a  change  in  the  high-frequency  resistance  would  be  expected  if  we  changed 
the  properties  of  that  surface. 


\ 

CALCULATED-1' 


W±Vl  LESSIH         III  KETIftS 


Fig.  1. 

The  experiments  described  below  were  undertaken  in  order  to  deter- 
mine whether  such  a  change  of  resistance  due  to  a  gas  skin  could  be 
detected.  The  effect,  if  it  exists,  would  be  especially  marked  in  some 
material  which  is  known  to  adsorb  gases  strongly.  Now  Merton1  has 
found  that  "copper  strongly  absorbs  most  gases."  I,  therefore,  formed 
a  square  loop  of  copper  wire,  enclosing  it  in  a  glass  tube  so  that  the 
conation  of  the  wire  surface  could  be  altered  at  will  and  measured 
the  current  flowing  in  the  wire.  A  change  in  the  resistance  in  the  circuit, 
if  capable  of  detection  under  the  limitations  of  these  experiments  would 
then  be  indicated  by  a  change  in  the  reading  of  the  galvanometer,  since 
the  electromotive  force  induced  in  the  circuit  was  maintained  constant. 

TEST  CIRCUIT. 

The  copper  wire  0.045  cm-  m  diameter  and  about  240  cm.  long  was 
mounted  at  the  center  of  a  glass  tube.  The  tube  was  made  in  two  L 
sections  which  were  so  placed  together  as  to  form  a  square  64  cm.  on 
each  side,  and  were  interconnected  to  the  vacuum  system  as  shown  in 
Fig.  2.  At  the  corner  of  each  L  tube  the  wire  was  supported  by  a  Pyrex 

1  T.  R.  Merton,  Chem.  Soc.  Jour.,  105,  645  (1914). 


VOL.  XX. 
No.  2. 


HIGH-FREQUENCY   RESISTANCE   OF   COPPER    WIRE. 


156 


glass  insulator  /,  connected  at  its  far  end  to  a  spring  5,  which  extended 
out  into  a  small  side  arm  which  made  equal  angles  with  the  sides  of  the 
L.  By  this  means  the  wire  was  kept  taut. 


(HE 


Fig.  2. 

At  the  ends  of  each  L  the  wire  was  attached  to  a  piece  of  1.05  mm. 
platinum  wire  and  sealed  out  through  the  end,  so  that  two  of  the  corners 
of  this  square  could  be  used  for  connecting  additional  apparatus.  At  one 
of  these  corners  a  single  plate  variable  air  condenser  C  was  connected, 
and  at  the  other  corner  an  arrangement  was  made  for  connecting  in 
various  standard  high  frequency  resistances,  R,  by  means  of  mercury 
cups.  Thus  the  resistance  of  the  circuit  could  be  measured  by  adding 
into  the  circuit  known  resistances. 

MEASURING  CIRCUIT. 

To  determine  the  change  in  current  in  the  test  circuit  produced  by 
different  conditions  of  the  wire,  a  circuit  containing  a  20  cm.  loop  of 


'57 


AUSTIN   BAILEY. 


[SECOND 

LSERIES. 


wire  closed  at  its  end  by  a  thermoelement  was  inductively  coupled  to 
this  circuit.  The  thermo-couple  was  especially  designed  and  constructed 
so  as  to  have  as  small  currents  as  possible  induced  in  the  galvanometer 
circuit.  This  was  done  by  making  the  plane  of  the  loop  at  right  angles 
to  the  galvanometer  connections  and  by  twisting  together  the  insulated 
leads  to  the  galvanometer  as  shown  in  Fig.  3.  The  copper-cons  tan  tan 
element  itself  was  mounted  in  a  bulb  which  was  evacuated  to  reduce 
thermal  losses.1  This  increases  the  sensitiveness  of  the  couple.2  The 
measuring  circuit  being  very  sensitive  could  be  loosely  coupled  to  the 
test  circuit  and  thus  introduce  only  a  very  small  resistance  into  the 
latter.  At  the  same  time  it  could  be  so  placed  that  its  loop  plane  was 
perpendicular  to  that  of  the  oscillator  circuit  and  thus  reduce  to  a 
minimum  direct  action  from  the  source  of  the  power. 

OSCILLATOR. 

A  three-electrode   vacuum  tube  connected  as  shown  in  the  diagram 
(Fig.  4)  was  used  as  a  high-frequency  generator. 


Fig.  4. 

v 

From  the  diagram  it  can  be  seen  that  the  oscillating  circuit  contains 
only  the  capacity  of  the  tube,  in  series  with  a  blocking  condenser  C', 
of  J  juf.  and  the  inductance  of  a  loop  of  wire  100  cm.  square,  L.  The 
hot-wire  ammeter  M  is  pla(ed  at  a  current  loop  so  that  it  will  indicate 
that  the  circuit  is  oscillating.  The  chokes  Ch  were  so  adjusted  that  they 

1  I.  Klemencic,  Wied.  Ann.,  42,  416  (1891). 

!  P.  Lebedew,  Ann.  d.  Phys.,  4  s.,  Q,  209  (1902). 


VOL.  XX. 1 
No.  2. 


HIGH-FREQUENCY   RESISTANCE   OF    COPPER    WIRE. 


158 


were  resonant  at  a  frequency  slightl>  greater  than  the  frequency  gen- 
erated by  the  oscillating  circuit.  Such  an  adjustment  makes  the  chokes 
have  a  high  inductive  reactance  at  the  frequency  used,  and  prevents  the 
high-frequency  current  from  going  back  into  the  direct-current  circuits. 
A  negative  potential  of  about  100  volts  is  maintained  on  the  grid  by  means 
of  the  grid-leak  resistance  R.  Any  desired  potential  on  the  plate  could 
be  obtained  by  means  of  a  potentiometer  contact  on  a  resistance,  through 
which  the  current  from  a  3oo-volt  generator  was  maintained  constant 
by  means  of  a  ballast  lamp.  This  circuit,  with  the  exception  of  the 
power  supply,  was  mounted  on  a  movable  base,  so  that  the  angle  of  the 
loop  with  reference  to  the  test  circuit  could  be  altered  by  rotation  about 
two  axes.  The  coupling  could  also  be  changed  by  altering  the  position 
of  the  oscillator  with  reference  to  the  test  circuit. 

MEASUREMENT  OF  FREQUENCY. 

A  wave-meter  was  made  of  a  variable  air  condenser  with  a  maximum 
capacity  of  670  wL  and  a  single  loop  of  No.  10  B.  &  S.  gauge  copper 
wire  20  cm.  in  diameter.  The  resonance  point  was  indicated  by  a 
hot-wire  galvanometer  inductively  coupled  to  this.  By  courtesy  of  the 
Commanding  Officer,  Radio  Laboratories,  Camp  Alfred  Vail,  New  Jersey, 
use  was  made  of  the  Lecher  system  of  parallel  wires,  constructed  there 
by  the  author  during  the  war,  for  the  purpose  of  calibrating  the  wave 
meter.  In  Fig.  5  is  shown  the  arrangement  of  the  circuit.  The  single- 


Fig.  5- 

plate  variable  air  condenser  C  was  adjusted  so  that  resonance  with  an 
outside  source  was  obtained  in  the  loop  circuit.  The  resonance  point 
was  very  sharp,  due  to  the  low  resistance  of  the  circuit,  and  was  indicated 
by  placing  the  hot-wire  galvanometer  at  the  point  B.  By  this  method 
the  parallel  wires  were  directly  coupled  to  a  circuit  of  fixed  frequency  and 
standing  waves  were  produced  on  them.  The  wires  were  very  long  in 
comparison  with  the  wave-lengths  to  be  measured. 

To  measure  the  wave-length,  a  wire-bridge  mounted  on  a  long  insulat- 
ing handle  was  slid  along  the  parallel  wires  until  the  hot-wire  galvanometer 
at  B  showed  a  sudden  decrease  in  reading  to  about  half  of  its  former 


I59  AUSTIN   BAILEY. 

value.  Suppose  this  position  was  A,  then  the  distance  AB  is  some  num- 
ber of  half  wave-lengths.  If  the  position  A  is  the  first  such  position 
found  after  leaving  B,  then  AB  is  one  half  wave-length.  A  large  number 
of  current  loops  could  be  located  in  this  manner,  but  since  each  position 
was  more  accurate  than  required,  only  two  such  determinations  were 
made.  While  the  method  employed  to  determine  the  wave-length  is 
similar  to  that  described  by  Rubens,1  the  use  of  a  small  hot-wire  gal- 
vanometer immediately  at  a  current  loop  to  indicate  the  positions  of 
other  current  loops  is  believed  to  be  of  considerable  advantage. 

With  an  oscillating  vacuum  tube  used  to  excite  the  system,  the 
positions  of  A  could  easily  be  located  so  that  the  value  of  the  wave- 
length thus  determined  was  accurate  within  0.5  per  cent.  The  accuracy 
of  this  method  for  giving  the  correct  indication  of  wave-length  was 
checked  down  to  38  meters  wave-length  by  a  wave-meter  composed  of 
a  standard  inductance,2  and  a  variable  air  condenser,  both  calibrated 
by  the  Bureau  of  Standards. 

VACUUM  SYSTEM. 

An  oil  pump  which  was  capable  of  reducing  the  pressure  to  about  I 
mm.  of  Hg  was  connected  through  a  three-way  stopcock  to  a  drying 
tube  containing  P2O5.  To  the  other  end  of  the  drying  tube  there  was 
connected  a  system  of  two  mercury  condensation  pumps  in  series. 
These  pumps  were  made  of  Pyrex  glass  and  connected  by  means  of  graded 
glass  seal 3  to  the  McLeod  gauge4  and  mercury  trap.  The  design  of  the 
pumps  is  shown  in  Fig.  6.  An  arc5  was  used  as  the  source  of  the  vapor 
which  was  then  conducted  to  the  condensation  chamber  through  a 
half-inch  tube  wound  its  entire  length  with  No.  18  B.  &  S.  gauge  soft 
iron  wire,  with  a  cord,  soaked  in  water-glass,  wound  between  turns  for 
insulation.  To  reduce  the  heat  loss  a  covering  of  woven  asbestos  tape 
was  wound  on  outside  of  the  wire  layer.  The  iron  wire  keeps  the  tube 
conducting  the  vapor  warm6  and  at  the  same  time  acts  as  a  ballast 
resistance  for  the  arc.  Tests  made  with  this  pump  showed  that  its 
maximum  rate  of  pumping  was  obtained  when  it  consumed  300  watts 
and  that  the  way  in  which  this  power  was  applied  made  no  difference, 
i.e.,  it  could  all  be  put  into  the  arc,  using  external  resistance,  or  it  could 
be  put  into  the  arc  and  the  resistance  wound  around  the  tube  conducting 

1  H.  Rubens,  Wied.  Ann.,  42,  154  (1891). 

2  Circular  74  of  the  Bureau  of  Standards,  page  320. 

3  W.  C.  Taylor  and  Austin  Bailey,  Jour.  Ind.  Eng.  Chem.,  13,  1158  (1921). 

4  Austin  Bailey,  PHYS.  REV.,  15,  319  (1920). 

6  L.  T.  Jones  and  H.  O.  Russell,  PHYS.  REV.,  10,  301  (1917). 
6  Gaede,  Ann.  Phys.,  4,  46,  357  (1915). 


VOL.  XX. 
No.  2. 


HIGH-FREQUENCY   RESISTANCE   OF   COPPER    WIRE. 


1 60 


the  mercury  vapor  to  the  condensation  chamber.  The  latter  method 
of  applying  the  power  was,  however,  found  to  have  the  distinct  advantage 
of  making  the  pump  effective  sooner  after  closing  the  switch  and  at  the 
same  time  of  requiring  a  smaller  current  for  operation. 


Fig.  6. 

OBSERVATIONS  AND  CONCLUSIONS. 

With  the  apparatus  arranged  as  described  above,  I  first  removed 
from  the  surface  of  the  wire  and  the  walls  of  the  containing  glass  tube 
much  of  the  adsorbed  gas.  This  was  accomplished  by  placing  around 
the  tube  containing  the  wire  an  electrical  resistance  furnace  and  heating 
the  glass  to  about  250°  C.,  while  at  the  same  time  the  wire  was  raised 
to  a  much  higher  temperature  by  passing  a  current  of  about  2  amperes 
through  it.  This  boiling-out  process  was  continued  for  from  three  to 
four  hours  and  then  the  tube  containing  the  wire  was  sealed  off  from 
the  pumping  system.  I  then  removed  the  furnace  from  the  wire  and 
allowed  the  apparatus  to  cool  to  the  temperature  of  the  room. 

At  the  end  of  this  process  the  surface  of  the  wire  was  apparently 
covered  with  a  layer  of  oxide.  To  remove  any  adsorbed  gas  from  this 
surface  layer,  the  wire  was  heated  as  hot  as  possible  without  burning 
out,  and  the  observations  begun  immediately  afterwards.  The  observa- 


AUSTIN   BAILEY. 


f SECOND 
LSERIBS 


tions  were  continued  until  there  appeared  to  be  no  further  change  in  the 
resistance  of  the  wire.  Expansion  of  the  wire  caused  by  the  increase  in 
temperature,  temporarily  changed  the  inductance  of  the  circuit.  To 
compensate  for  this  change,  the  circuit  was  constantly  retuned  during 
the  cooling  of  the  wire  by  adjusting  the  variable  condenser  C  shown  in 
Fig.  2.  This  adjustment  was  made  at  a  distance  from  the  circuit  by 
means  of  a  cord  wound  around  the  condenser  handle,  which  enabled 
the  observer  to  tune  accurately  without  influencing  the  circuit  by  the 
close  proximity  of  his  body.  The  position  of  the  observers  remained 
fixed  during  each  set  of  observations. 

The  attached  curves  show  the  results  of  several  such  runs.  Fig.  7 
shows  successive  runs  on  a  wire  which  had  previously  been  boiled  out 
as  described  above.  Two  heatings  of  the  wire  to  red  heat  were  made 
to  drive  off  adsorbed  gas.  This  raised  the  pressure  in  the  tube  to  about 
4  X  io~3  mm.  of  Hg  as  indicated  by  the  appearance  of  the  discharge 
through  the  tube  from  a  spark  coil.  One  day  later  the  wire  was  again 
heated  red  hot  and  beginning  twenty  minutes  later  the  series  of  observa- 
tions shown  in  Fig.  7  were  made.  The  initial  decrease  in  the, resistance 


3:00        TU1 


Fig.  7. 


of  the  wire,  shown  by  the  rising  portion  of  the  curves,  is  obviously  due  to 
the  cooling  of  the  wire,  for  the  tests  made  of  the  time  required  to  obtain 
a  maximum  reading  with  the  temperature  of  the  test  circuit  constant 
proved  that  this  gradual  increase  could  not  be  due  to  the  sluggishness  of 
the  measuring  circuit.  The  subsequent  decrease  of  reading  is  attributed 
to  the  readsorption  of  a  film  of  gas  by  the  oxide  surface.  As  this  takes 
place  slowly  at  low  pressures  the  galvanometer  deflections  go  through  a 
maximum,  showing  that  the  wire  has  a  decreased  resistance  for  a  few 


VOL.  XX.l 
No.  2. 


HIGH-FREQUENCY   RESISTANCE   OF   COPPER    WIRE. 


162 


minutes  after  the  wire  has  cooled  and  before  the  gas  has  had  time  to 
be  completely  readsorbed.  The  progressive  decrease  in  the  ordinates 
of  successive  curves  in  the  series  shown  in  Fig.  7  is  probably  due  to  the 
discharge  of  the  storage  battery  of  40  amp. -hours'  capacity  which  was 
running  the  motor-generator  and  discharging  at  an  8  amp.  rate.  The 
increase  in  the  brightness  of  the  appearance  of  the  wire  after  successive 
heatings  indicated  that  the  flattening  out  of  the  maximum  was  probably 
due  to  the  removal  of  the  oxide  covering,  thus  decreasing  the  adsorbing 
surface. 

The  same  wire  used  in  Fig.  7  was  heated  in  the  resistance  furnace 
again  for  four  hours  at  about  250°  C.  and  the  appearance  of  the  wire 
surface  changed  from  a  bright  copper  color  to  a  reddish  brown,  showing 
that  the  oxide  layer  which  had  been  decomposed  by  the  heatings  pre- 
viously given  to  it  had  been  partially  reformed.  The  behavior  of  this 
wire,  plotted  in  Fig.  8,  was  similar  to  that  shown  in  Fig.  7.  No  quantita- 


|    KIM  BCRSIO 

I  ow  tm 

BEIRS  HIITED 


TIMS   OP   COOLIHG 

Fig.  8. 

tive  comparison  can  be  made  between  these  two  figures  or  those  following 
because  of  changes  made  in  the  position  of  the  measuring  circuit.  Un- 
fortunately the  wire  burned  out  during  the  third  heating  and  a  complete 
series  could  not  be  obtained. 

The  curves  plotted  in  Figs.  9  and  10  are  made  with  a  different  wire. 
Previous  to  the  first  curve  plotted  in  Fig.  9  the  wire  had  been  boiled  out 
and  sealed  off  in  a  tube,  and  had  been  allowed  to  stand  several  days. 
The  pressure  in  this  tube  was  not  as  low  as  in  the  tube  previously  used. 
An  interval  of  7|  hours  elapsed  before  the  first  curve  in  Fig  9, 
during  which  time  the  storage  battery  was  recharged.  The  curve  follow- 
ing this  charge  of  the  storage  battery  is  very  erratic  and  full  of  accidental 
variations  but  it  shows  strong  indication  of  a  maximum.  The  next  curve 


1 63 


AUSTIN   BAILEY. 


[SECOND 

[SERIES. 


is  more  consistent  in  its  points  and  fewer  abrupt  changes  in  galvanometer 
deflections  were  observed. 

On  allowing  the  air  to  again  fill  the  tube  at  atmospheric  pressure  no 
marked  change  occurred.     (See  Fig.  9.)     After  heating  the  wire  in  air 


EElSi 


irtni 


Fig.  9. 

for  ten  minutes  its  surface  appeared  almost  black,  due  undoubtedly  to 
the  formation  of  cupric  oxide,  CuO.  Immediately  after  the  heating 
current  was  turned  off  the  oscillator  was  started  again  and  observations 
made  of  the  galvanometer  deflections.  These  show  a  slight  indication 
of  a  maximum.  If  this  is  a  true  maximum  it  is  probably  due  to  the  same 
cause  as  before  but  is  much  sharper  because  the  wire  was  initially  at  a 
lower  temperature,  could  cool  more  quickly,  and  would  probably  readsorb 
gas  more  rapidly  at  atmospheric  pressure. 

The  oscillator  w^as  again  stopped,  the  tube  connected  to  the  vacuum 


Fig.  10. 


NoL'2XX']          HIGH-FREQUENCY   RESISTANCE   OF   COPPER    WIRE.  164 

system  and  the  pressure  reduced  to  3  X  io~4  mm.  of  Hg.  The  next 
two  curves  (Fig.  10)  taken  while  the  pumps  were  running,  shows  no 
maximum,  probably  because  at  this  low  pressure  the  gas  was  readsorbed 
more  slowly  by  the  oxide  layer  after  heating.  After  the  second  heating 
at  this  low  pressure  the  wire  was  apparently  covered  with  the  red  oxide, 
Cu2O.  When  the  mercury  condensation  pumps  were  turned  off  and  the 
backing  pump  still  kept  operating,  the  pressure  gradually  increased. 
The  mercury  in  the  pumps  was  so  hot  that  it  continued  to  go  over  and 
condense  for  some  time  after  the  current  was  turned  off.  This  continued 
operation  of  the  mercury  pumps  thus  allowed  the  pressure  to  increase 
gradually.  The  first  abrupt  rise  of  the  third  curve  shown  in  Fig.  10 
may  be  due  to  some  cause  connected  with  stopping  the  operation  of  the 
mercury  pumps,  since  the  circuit  at  this  frequency  is  very  subject  to 
variations  from  slight  changes  in  nearby  circuits.  The  final  decrease 
of  the  galvanometer  deflection  however  is  probably  due  to  the  readsorp- 
tion  of  the  gas  as  the  pressure  increased. 

Any  single  curve  taken  by  itself  would  probably  have  little  or  no 
significance  owing  to  the  large  fluctuations  in  the  readings,  due  to 
accidental  influences,  but  all  the  curves  taken  together  strongly  indicate: 
(i)  that  gas  is  readily  adsorbed  by  copper  oxide,  and  is  easily  liberated 
by  heating,  (2)  that  copper  either  does  not  adsorb  gases  as  readily  or 
that  the  adsorbed  layer  is  not  as  easily  liberated  from  the  copper  by 
heating,  (3)  that  the  resistance  of  the  wire  to  high  frequency  currents 
is  increased  by  the  presence  of  the  gas  skin. 

The  "precipitated  copper"  as  used  by  Merton  was  doubtless  largely 
cuprous  oxide  as  he  describes  it  as  "very  dark  brown  in  color,"  and  as 
"prepared  by  reducing  a  solution  of  the  copper  salt."  As  is  well  known 
Cu2O  is  prepared  in  this  way.  Finely  divided  copper  can  also  be  pre- 
pared in  this  way  by  the  addition  of  suitable  reducing  agents  such  as 
N2H4H2O.  His  statement  that  "if  the  copper  is  too  strongly  heated, 
it  undergoes  a  change  in  color  and  loses  its  absorbing  power,"  would  also 
tend  to  show  that  the  "precipitated  copper"  was  largely  Cu2O  and  that 
on  heating  too  strongly  it  underwent  a  chemical  change,  probably  be- 
coming metallic  copper.  Thus  interpreted,  his  results  are  in  entire 
agreement  with  the  conclusions  given  above.  - 

Copper  wire  when  used  commercially  is  doubtless  covered  with  a 
thin  layer  of  oxide,  which  strongly  adsorbs  gases,  and  which  therefore 
according  to  the  above  results  increases  the  resistance  of  the  wire. 
Covering  the  surface  of  the  wire  with  some  metal  which  does  not  easily 
oxidize  and  at  the  same  time  does  not  appreciably  adsorb  gases  would 
probably  decrease  the  resistance  of  the  wire.  Possibly  tin-covered 


165  AUSTIN   BAILEY. 

copper  wire  would     prove  to  have  a  lower  high-frequency  resistance 
than  copper  wire  of  the  same  size. 

In  closing,  I  wish  to  express  my  keen  appreciation  of  the  interest  shown 
and  encouragement  given  in  this  work  by  Professor  Ernest  Merritt  and 
of  the  cooperation  of  Professor  R.  C.  Gibbs  in  obtaining  the  material 
necessary  to  work  out  this  problem.  The  advice  and  suggestions  given 
by  other  members  of  the  Department  of  Physics  throughout  the  progress 
of  this  research  is  also  greatly  appreciated.  Likewise,  I  wish  to  acknowl- 
edge the  generous  assistance  which  I  have  received  from  Professor 
Ernest  Merritt  and  Doctor  Gordon  S.  Fulcher  in  preparing  this  material 
for  publication. 

CORNELL  UNIVERSITY, 
June,  1920. 


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